The goal of this project is to understand the mechanisms that regulate alternative splicing during striated muscle development. Cardiac troponin T (cTNT) undergoes developmentally regulated alternative slicing in striated muscle, and is a useful model for cell-specific splicing regulation. cTNT pre-mRNAs contain muscle-specific splicing enhancers (MSEs), intronic elements that are required for the enhanced alternative exon inclusion that is observed in embryonic striated muscle. ETR-3, a member of a conserved family of RNA-binding proteins, binds MSEs and promotes MSE-dependent exon inclusion. ETR-3 is induced during skeletal muscle differentiation and undergoes an isoform transition during heart development. Both of these changes correlate with regulated switches in cTNT splicing. These results strongly suggest that ETR-3 is a major regulator of alternative splicing in striated muscle development. To determine the role of ETR-3 in muscle-specific alternative splicing, I will: (i) Characterize ETR-3 as a positive regulator of alternative splicing during skeletal muscle differentiation, (ii) Determine the significance of the ETR-3 isoform transition during heart development, and (iii) Determine the role of ETR-3 in coordinated regulation of alternative splicing during striated muscle development. ETR-3 is 78% identical to CUG-BP, which is proposed to mediate pathogenesis of myotonic dystrophy, an autosomal dominant genetic disorder caused by an expansion of unstable CTG repeats in the 3' UTR of the DMPK gene. Expanded DMPK transcripts accumulate in the nucleus, and are thought to affect post-transcriptional processing of other genes by disrupting function of CUG-BP. Our results suggest that ETR-3 is an equally likely candidate for mediating a trans-dominant effect in DM.